Gene Interaction and Continuous Traits 



59 



extreme (1.75 units). These results typify 

 an effect of dominance called regression, as 

 a consequence of which an individual pheno- 

 typically extreme in either direction will have 

 progeny less extreme. 



Figure 8-3 illustrates the principle of re- 

 gression. Had no dominance obtained the 

 average offspring from parents at A, B, and C 

 would have been at the corresponding points 

 A', B', C, respectively, in the offspring curve. 

 (The environment would have been the cause 

 of some phenotypic fluctuation around these 

 mean points in the offspring curve.) In the 

 case of dominance, however, the offspring 

 of A would be, on the average, to the right 

 of A, as shown by arrows, while the offspring 

 of C would be, on the average, to the left of 

 C. The loss of extreme individuals genera- 

 tion after generation would not make the 

 entire population more and more homo- 

 geneous phenotypically, however, since there 

 would be an exactly counterbalancing tend- 

 ency from the average, C, members of the 

 population to produce offspring more ex- 

 treme than themselves in either direction. The 

 result, as in cases of no dominance, is that 

 the distribution curve for the offspring would 

 be the same as for the parent population. 



Now, suppose a population of individuals 

 showed a quantitative character and we 

 wanted to obtain a line of them which was, 

 on the average, say, larger. We would use 

 the largest individuals as parents (Figure 

 8-4). Had the genes involved shown no 

 dominance then the very first offspring gen- 

 eration would have the same mean as the 

 group selected as parents. But since dom- 

 inance usually obtains, regression will occur, 

 and the mean size of the first generation of 

 offspring will be somewhat less than that of 

 the selected parents but somewhat more than 

 the original mean. By continuing to select the 

 largest individuals to serve as parents over a 

 number of generations, the offspring of suc- 

 ceeding generations will approach closer and 

 closer the size of the selected parents. 



PARENTS 



7^>^r><r\ 



OFFSPRING 



FIGURE S-3 (Above). T/ie principle of regression. 



FIGURE S-4 (Below). Selection for a quantitative 

 character. 



PARENT 

 GENERATION 



Group Selected 

 as Parents 



Mean 



Mean of 

 selected group 



OFFSPRING 

 GENERATION 



Mean 



